ALSA sometimes shows channels of which you can not change the volume. If you
unmute those channels, it'll force the regular channels to not provide any sound
anymore.
The user must leave these channels muted.
Thanks to Rickard Lindgren <r.lindgren@kolumbus.fi> for noticing, confirmed by
Martin Schmeisser. The reason: most of the time, these channels provide a
different output; enabling them disables the normal output. For instance, IEC985
unmuting disables regular output in favor of S/PDIF.